For seven years, Michael Polsky has been trying to build a high-voltage power line from the windswept prairies of Kansas to the ravenous electrical grids of the Midwest and East Coast. He brings plenty of firepower to the task. His Chicago-based company Invenergy is the country’s largest privately held generator of renewable power. And his investors include Blackstone, the world’s biggest private equity firm. But it’s been a seesaw battle, and the outcome is still in doubt. One of the main problems for Polsky’s power line is that it needs to cross lots of farmland in Kansas, Missouri and Illinois....